For current rotomolding fuel tank technology, single layer (typically 2.5 to 20 mm thick) crosslinked polyethylene is commonly used for the production of fuel tanks, chemical vessels and many other containment-type products. This material is widely known as having very tough and durable qualities that allow it to be used in such demanding applications. Also, the rotational molding process allows very complex shapes to be manufactured from these types of materials. In many cases, rotational molding is the only plastics production method that could be used to economically produce such complex shapes. However, crosslinked polyethylene does allow the permeation of hydrocarbons and other fluids and vapors at rates that do not conform to some current and future environmental regulations e.g. California Air Resource Board (CARB) and the Environmental Protection Agency (EPA). Therefore, there is a need to develop a lower permeation material for the rotational molding process that meets industry standards and has suitable mechanical properties for these applications.
The production of rotationally molded products with two layers of materials is common practice in the industry today. Typical material combinations include polyethylene and polyurethane, polyethylene and polyethylene, polyethylene and polyethylene with a foaming agent, polyamide 11 and grafted polyethylene; to produce a structure that benefits from the combination of materials being used. This process typically uses a dropbox, which is attached to the mold, to store and then release the second charge of material into the mold at a critical temperature during the molding cycle.
Arkema Incorporated has developed a low permeation fuel tank solution using PetroSeal™ technology (reference International Publication No. WO 2004/045849 A1). This technology uses two layers of material, Rilsan® polyamide 11 and a grafted polyethylene, to produce a material combination that can be used by the rotational molding process to produce a vessel or tank with low permeation properties. However, Polyamide 11 lacks low temperature dart impact strength, which may limit the applications it can be used for. The cost of Polyamide 11 may also be a limiting factor.
U.S. Pat. No. 6,946,176 B2 refers to the development of a multi-layer thermoplastic structure that can be produced by the rotational molding process. This technology can be used to produce a bladder with a thermoplastic barrier material with a high viscosity (such as ethylene vinyl alcohol), enclosed between two layers of thermoplastic materials (such as linear polyethylene or polyamide). The intended application for this combination of materials is to produce a multi-layer bladder for high-pressure gas tanks or bottles.
For other plastics processes such as blow molding, thermoforming and injection molding, thermoplastic materials such as polyethylene, ethylene-vinyl alcohol, polypropylene, polybutene, polybutylene terephthalate, polyethylene terephthalates, or polyamide 6, 11 and 12 can be used to produce a low permeation vessel or container. In particular, blow molding uses a multi-layer system using virgin polyethylene, regrind polyethylene, adhesive, ethylene-vinyl alcohol, adhesive, and virgin polyethylene. This is a commonly accepted method to produce a seamless, low permeation fuel tank using the blow molding process. Thermoforming has the ability to use this combination of materials in pre-formed sheets which can then be heated and used to form two halves of a vessel or tank, which can then be joined together while still hot to produce a sealed object with low permeation properties. This is referred to as twin-sheet thermoforming. Injection molding can also use some of the materials previously listed, however the process to manufacture a vessel or a tank requires that two or more molded pieces must be formed and then later fused or welded together, in order to make the structure leak-proof. A low permeation vessel with high impact and moisture resistance that can be formed by rotomolding is needed to meet increasing standards for environmental protection.